The present invention relates to an electronically controlled throttle device for electrically controlling the quantity of air intake to a vehicle-mounted engine, and more particularly to an electronically controlled throttle device suitable for a diesel engine.
An electronically controlled throttle apparatus (electronic throttle apparatus) which controls the quantity of air intake into a gasoline engine by optimally driving an actuator (e.g. a DC motor, torque motor, or stepping motor) are already in use. Such an throttle apparatus controls the position of the throttle valve with an actuator so as to make it identical with a target opening computed according to the treading depth of the accelerator pedal or the operating state of the engine. And the throttle apparatus detects its behavior with a throttle position sensor, and corrects the position under feedback control.
Conventional electronic throttle apparatuses include a structure which, as described in the Japanese Patent Laid-Open No. H10(1998)-30675 for instance, is provided with a drive mechanism equipped with an actuator for controlling the throttle valve position and a throttle position sensor for detecting the throttle valve position, which are arranged in a sealed space, with the wiring for the sensor and the actuator being integrated.
The other throttle apparatuses for controlling the throttle position also include one by which, as described in the Japanese Patent Laid-Open No. H7(1995)-332136 for instance, a control quantity corresponding to the deviation of the actual opening of the throttle valve from the target opening is computed by PID control or a similar technique. The computed control quantity is converted into a duty ratio, which is the ratio between the on time and the off time of pulse driving, a PWM signal is supplied to a DC motor via an H bridge circuit. The motor generates torque, and the throttle valve is driven by the generated torque via a gear and a throttle shaft to control the position.
The Electronic throttle apparatuses described above are generally used for gasoline engines. Recently, electronic throttle apparatuses are beginning to be applied to diesel engines with a view to enhancing the EGR efficiency and improvement in dieseling. Since electronic throttle apparatuses for diesel engines, unlike those for gasoline engines, perform control to enhance the EGR efficiency or to burn soot in the DPF (diesel particulate filter). The DPF is performed by raising the exhaust temperature by throttling the air intake. In diesel engines, the motor control is stopped when neither EGR control nor DPF control is performed, and the throttle valve is in its full open position. Accordingly, they are significantly different from gasoline engines in that 1) the full open position is maintained for a long period, 2) there is a transition from the active state of motor control to its stopped state or a transition vice versa, and 3) Since there is no runaway mode, a default mechanism, which holds any arbitrary degree of opening for supplying a constant quantity of air when power supply to the motor is off, is unnecessary.
In electronic throttle apparatuses for diesel engines, when EGR control or DPF control has ended, there is no need to control the air flow rate by throttle valve like gasoline engines. Therefore, when power supply to the motor is turned off, the throttle valve is returned by a return spring to the full open position, thereby the pressure loss of the air intake is least. Thus, unlike electronic throttle apparatuses for gasoline engines which perform control all the time, they always have a transition from the active state of motor control to its stopped state or a transition opposite to it.
To begin with, considering a transition from the active state of motor control to its stopped state, a first problem is as follows. When power supply to the motor is simply turned off or the provided duty is reduced to 0% at the time of stopping the control, the throttle valve position is returned to the full open position by a return spring rapidly. Then the full open stopper of the throttle valve and drive mechanism parts will violently clash with each other, inviting problems of collision noise and an effect of the impact load to shorten the service life of mechanical parts.
As an attempt to cope with this problem, an electronic throttle apparatus which is provided with a buffer mechanism between the fully open stopper and gears to mechanically prevent from collision is known, as described in the Japanese Patent Laid-Open No. 2002-256892 for instance.
In another known electronic throttle device described in the Japanese Patent Laid-Open No. 2003-214196, for instance, it is intended to prevent from the collision by supplying a preset predetermined value (power), which drives the motor at a lower speed than under normal control, to the motor for any arbitrary length of time.
However, the formula described in the Japanese Patent Laid-Open No. 2002-256892 involves the problems of the additional cost of the buffer mechanism, the reduced effect of a deteriorated buffer mechanism, and a loss in reliability due to the increased number of components.
On the other hand, in the formula described in the Japanese Patent Laid-Open No. 2003-214196, because it is to perform control by supplying a preset predetermined value to the motor for any arbitrary length of time, there are differences among individual products in response time and other respects. Therefore, it has the possibility that the motor may continue to drive even if the throttle valve returns to its full open position, and an excess current may damage the motor or a consequent overload may work on and damage mechanical parts.